Dimmer function for EL displays

ABSTRACT

A control circuit to vary the intensity of an electro luminescent display. The circuit is connected between a power source and a display and comprises a pair of conductors to be connected to the display for applying a voltage from a voltage generator. A switch controls application of the voltage from the generator to the conductors. A gating circuit connects selectively one or other of the conductors to the voltage source. A controller operates upon the switch to vary the duty cycle and upon the gating circuit to alternate periodically the relative polarity of the conductors. The voltage source includes an inductor and the switch controls current flow through said inductor.

FIELD OF THE INVENTION

The present invention relates to a dimmer function for use with adisplay device such as an EL (electroluminescence) display.

BACKGROUND OF THE INVENTION

EL displays are frequently used to display images such as graphics, textand other types of visual information under ambient light conditionswhich can vary greatly. In high intensity ambient light conditions, itmay become difficult to properly view the images displayed on the ELdisplay. As well, under low ambient light conditions, the imagesdisplayed on the EL display may be overly bright.

Accordingly, it is helpful if the EL display is controlled by a dimmerso that image brightness may be increased when the intensity of theambient light is high. When the intensity of ambient light is low, thebrightness of the image may be decreased. The intensity of the displayis a function of the maximum voltage applied and in the art the voltageis controlled by a silicon controlled rectifier (SCR). Control of SCR isdifficult and has a significant power consumption. Moreover, withportable devices the battery voltage used as a power source will alsovary over time and accordingly the intensity of the display tends tofluctuate with the battery voltage.

Accordingly, it is an object of the present application to obviate ormitigate the above disadvantages.

SUMMARY OF THE INVENTION

The present invention seeks to provide a solution to the problem ofmaintaining and adjusting the intensity of a display as a power sourcevaries.

In one aspect, the present invention provides a control circuit to varythe intensity of an electro luminescent display. The circuit isconnected between a power source and a display and comprises a pair ofconductors to be connected to said display for applying a voltagethereto, a voltage generator, a switch to control application of saidvoltage from said generator to said conductors, a gating circuit toconnect selectively one or other of said conductors to said voltagesource, and a controller operating upon said switch to vary the dutycycle thereof and upon said gating circuit to alternate periodically therelative polarity of said conductors.

An embodiment of the invention will now be described by way of exampleonly with reference to the following detailed description in whichreference is made to the following appended drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a hand held scanner,

FIG. 2 is a schematic diagram of a dimmer circuit utilized in thescanner of FIG. 1.

FIG. 3 is a simplified line diagram showing a period of an example of apulse line used by the dimmer circuit.

FIG. 4 is a simplified line diagram showing an example of the chargingand discharging cycles in relation to a pulse line used by the dimmercircuit.

FIG. 5 illustrates a generalized flow chart of an algorithm to producethe charging/discharging pulse.

FIG. 6 illustrates a generalized flow chart of an algorithm to producethe charging/discharging pulse and the synchronization of the H-bridgeoperations.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a hand held scanner 2 having a body4 and a display 14. The scanner may include an input device, such askeypad 6, and is used to read and store information from barcodes or thelike through a scanner window 8. The body 4 contains control and dataacquisition components as well as a voltage source 9 (FIG. 2) to supplypower to the device. The scanner 2 may be used in a variety ofenvironments in which different levels of intensity for the display 14are desirable.

The keypad 6 includes a manual dimmer control 7 for use in setting adimmer circuit 10 controlling the amplitude of voltage applied to an ELdisplay 14 through a pair of conductors or lines 16, 18 shown in FIG. 2.The dimmer circuitry 10 includes an inductor 114, that is supplied withpower from the voltage source 9 at a nominal voltage. A N-chanel FET 110controls current flow through the inductor 114, which is connected inparallel to an H-bridge 19 having four legs 20, 22, 24, 26. One pair oflegs 20, 22 controls the voltage to line 16 connected to display 14 andthe other pair 24, 26 controls the voltage applied to line 18 connectedto display 14. The voltage applied across the lines 16, 18 determinesthe intensity of the display 14. The voltage in lines 16, 18 is alsocontrolled by FETs 170, 150 respectively.

Each of the legs 20, 22, 24, 26 includes an N-chanel FET 120, 140, 130,160 respectively. Current flow between legs 20, 22 and 24, 26 iscontrolled by diodes 142, 162 and within each leg by resistors 124, 134,144, 164.

The operation of the H-bridge 19 is controlled through a microprocessor12, which interfaces with the main processor 30 of the scanner 2. Themicroprocessor 12 is grounded by line 280 and connected through line 220to voltage regulator 50. The N-chanel FETs 140, 150, 160, 170 areswitched by control lines 240, 250, 260, 270 respectively from themicroprocessor 12 to direct the inductor flyback energy into the ELdisplay 14 through lines 16, 18. The microprocessor 12 also controls thestate of N-channel FET 110 through control line 210 to regulate thecharging and discharging of the inductor 114.

The H-bridge 19 capacitors 126, 136 are used to store a charge so thatN-channel FETs 120, 130 may be turned ON. These capacitors 126, 136 arerequired to ensure that N-channel FETs 120, 130 maintain a positivevoltage from gate to source. This is due to the fact that the EL display14 is charged to a high voltage and that the gate voltage must begreater than the source voltage, but there are no voltage sources thatare higher in voltage than that impressed on the EL display 14.

Capacitor 126 is charged through H-bridge 19 diode 122 wheneverN-channel FETs 140, 150 are ON. Capacitor 136 is charged throughH-bridge 19 diode 132 whenever N-channel FETs 160, 170 are ON.Capacitors 126, 136 are initially charged up using the N-channel FETs140, 160 which are connected in series with H-bridge 19 resistors 144,164 just before starting the EL display 14 lighting sequence. Thepresence of resistors 144, 164 ensures that the current draw is notexcessive when initially charging the uncharged capacitors 126, 136.

The operation of the circuit of FIG. 2 will be described assuming thatcapacitors 126, 136 are fully charged. The microcontroller 12 thenselects a high leg and a low leg of H-bridge 19 by applying controlsignals so that N-channel FET 120 is turned ON by turning OFF N-channelFET 140, N-channel FET 130 is turned OFF by turning ON N-channel FET160, N-channel FET 150 is turned OFF and that N-channel FET 170 isturned ON. The microprocessor 12 turns ON N-channel FET 110 and currentflows through the inductor 114 to ground.

After a time determined by the microprocessor 12, N-channel FET 110 isturned OFF and current flow through inductor 114 to ground isinterrupted. The flyback energy from the inductor 114 causes current toflow through diode 112, N-channel FET 120, line 18, the EL display 14and N-channel FET 170. This causes the EL display 14 to become chargedto the voltage induced by the inductor 114. The N-channel FET 110 iscycled by the microprocessor 12 so that several inductor 114 flybackpulses are sent to the EL display 14 while the H-bridge 19 is held inthis state to increase the voltage applied across the lines 16, 18 untilit attains the level required for the selected brightness.

After the required number of pulses, the EL display 14 is discharged bythe microprocessor 12 turning ON the N-channel FET 140, which turns OFFN-channel FET 120, and leaving N-channel FET 170 ON. N-channel FET 140is used instead of N-channel FET 150 because it has a series resistor144 to limit the intensity of current flow. This helps minimize thepresence of high current pulses so that Electro-Magnetic Interference(EMI) may be reduced.

The H-bridge 19 is then turned around by the microprocessor 12 selectingthe controls lines so that N-channel FET 130 is turned ON, which turnsOFF N-channel FET 160, N-channel FET 150 is turned ON and N-channel FET170 is turned OFF. Several inductor flybacks are then sent to the ELdisplay 14, causing the EL display 14 to be charged to the oppositepolarity.

Once again, after the required number of pulses, the EL display 14 isdischarged by turning ON N-channel FET 160, which turns OFF N-channelFET 130, and leaving N-channel FET 150 ON. N-channel FET 160 is usedinstead of the N-channel FET 170 because it has a series resistor 164 tolimit the intensity of current flow. As previously mentioned, this helpsminimize the presence of high current pulses so that Electro-MagneticInterference (EMI) may be reduced, The H-bridge 19 is then turned aroundonce more: N-channel FET 120 is turned ON, which turns OFF N-channel FET140, N-channel FET 150 is turned OFF and N-channel FET 170 is turned ON.Several inductor flybacks are then sent to the EL display 14, causingthe EL display 14 to be charged to the opposite polarity.

The purpose of diodes 142, 162 is to clamp the gates of N-channel FETs120, 130 so that they do not become much more negative than the sourcevoltage 9 when N-channel FETs 120, 130 are turned OFF. If they becamemore negative by, for example 20V, the gates oxides of N-channel FETs120, 130 may be destroyed. Thus the microprocessor 12 provides a numberof pulses to the EL display 14 with the H-bridge 19 in oneconfiguration, discharges the EL display 14 and then provides a numberof pulses with the H-bridge 19 in an opposite configuration. The flybackvoltage caused by discharges of inductor 114 is applied to EL display 14to determine the intensity of the display.

The microcontroller 12 controls the charging and discharging of inductor114 by transmitting a pulse, on control line 210, to N-channel FET 110.By varying the charging and discharging durations, the intensity of theEL display 14 may also be varied. FIG. 3 shows an example of a period ofsuch a pulse line 100 where T is the period of the pulse 100 and T₁ isthe time the pulse 100 is HIGH. When the pulse 100 is HIGH, N-channelFET 110 is turned ON and inductor 114 is charged for T₁ seconds, then,when the pulse 100 is LOW, N-channel FET 110 is turned OFF and inductor114 is discharged for (T−T₁) seconds, which provides flyback energy tothe EL display 14. FIG. 4 shows an example of the charging anddischarging cycles of the EL display 14 in relation to a pulse 100 suchas illustrated in FIG. 3. During the charging cycle of the EL display14, when the pulse 100 is LOW, the flyback energy from the discharge ofinductor 114 causes a rise 102 in the charge of the EL display 14 andwhen the pulse is HIGH, the charge of the EL display remains constant104 while the inductor 114 is being charged. This stepwise chargingcontinues until a predetermined number of pulses per polarity P isattained, at which time the charge of the EL display 14 has reached therequired maximum value V_(EL) across lines 16, 18 corresponding to thedesired brightness level. The EL display 14 is then discharged 106, theH-bridge 19 is turned around so that the polarity is inversed, and theprocess is repeated. This process creates an alternating voltage acrosslines 16, 18 from voltage source 9.

Adjusting the duty cycle of the inductor 114 controls the amount ofenergy stored in the inductor 114 on each pulse. The duty cycle is thefraction of time the pulse 100 is HIGH during a pulse 100 period T₁ asdescribed by Equation 1. $\begin{matrix}{{{duty}\quad{cycle}} = \frac{T_{1}}{T}} & {{Equation}\quad 1}\end{matrix}$

The amount of energy stored during each pulse is proportional to theinductance and the square of the current, as described by Equation 2.$\begin{matrix}{E = \frac{{LI}^{2}}{2}} & {{Equation}\quad 2}\end{matrix}$

-   -   where L is the inductance value of inductor 114    -   I is the inductor's 114 current during the pulse.

This current may be expressed by Equation 3: $\begin{matrix}{I = \frac{{VT}_{1}}{L}} & {{Equation}\quad 3}\end{matrix}$

-   -   where V is the voltage of voltage source 9,    -   T₁ is the time the pulse 100 is HIGH.

Varying the duty cycle varies the rate at which energy is transferred tothe EL display 14, which therefore controls the power and hencebrightness. The power may be approximated by:P≈E·f  Equation 4

-   -   where E amount of energy stored during each pulse,    -   f is the frequency of the pulse 100.

Combining Equation 2, Equation 3 and Equation 4 gives Equation 5 inwhich the power is proportional to the square of the voltage of voltagesource 9, the period and the square of the duty cycle of pulse 100, andinversely proportional to the inductance value of inductor 114.$\begin{matrix}\begin{matrix}{P \approx {\frac{1}{2} \cdot L \cdot \left( {\frac{V}{L}T_{1}} \right)^{2} \cdot \left( \frac{1}{T} \right)}} \\{= {{\frac{1}{2} \cdot \frac{V^{2}}{L} \cdot \frac{T_{1}^{2}}{T}} = {\frac{1}{2} \cdot \frac{V^{2}}{L} \cdot \frac{{T^{2} \cdot D}\quad C^{2}}{T}}}} \\{= {{\frac{1}{2} \cdot \frac{V^{2}}{L} \cdot T \cdot D}\quad C^{2}}}\end{matrix} & {{Equation}\quad 5}\end{matrix}$where DC is the duty cycle of pulse 100.

Thus by varying the period and/or duty cycle the intensity of the ELdisplay 14 may be varied.

Pulse 100 is generated by alternately setting control line 210 to HIGHfor a duration of T₁ seconds and then to LOW for a duration of (T−T₁)seconds. The timing of the HIGH and LOW portions of the pulse 100 may beachieved by translating times T₁ and (T−T₁) into an equivalent number ofinstructions to be performed by the microcontroller 12. The number ofinstructions microcontroller 12 executes per second is determined bydividing its frequency by 4. For example, a microcontroller running offits internal 4 MHz clock executes 1,000,000 instructions per second or,in other words, one instruction is executed each 1 μs. Thus, correcttiming is achieved by executing a number of instructions equal to:$\begin{matrix}{{{number}\quad{of}\quad{instructions}} = \frac{T_{1}}{1\quad{µs}}} & {{Equation}\quad 6}\end{matrix}$for the HIGH pulse; and $\begin{matrix}{{{number}\quad{of}\quad{instructions}} = \frac{\left( {T - T_{1}} \right)}{1\quad{µs}}} & {{Equation}\quad 7}\end{matrix}$for the LOW pulse.

An example of an algorithm that may be executed by the microcontroller12 to obtain pulse 100 is depicted by the flow chart shown in FIG. 5. Inthis example, an 8-bit microcontroller 12 is used, such as, for example,the PIC12C508A, which has a 4 MHz internal clock. The sequence of stepscomposing the algorithm is indicated by the sequence of blocks 302 to314.

In block 302 the algorithm starts by setting the output of control line210 to HIGH.

At block 304, a counter is set so that the proper number of instructionsis executed in order to translate into the required time duration (e.g.T₁ or (T−T₁) seconds depending if the control line 210 is presently atHIGH or LOW respectively). In the case of an 8-bit microcontroller 12,there is a circular counter that is increased every time an instructionis executed. The counter increases from 0 to 255, at which time it goesback to 0 and repeats the cycle. Thus, to count N instructions, thecounter has to be set according to the following equation:counter value=256−N  Equation 8

If a duration of 50 μs is required, using either Equation 6 or Equation7, this translates into 50 instructions, the counter is then set to 206(256−50). Since the setting of the counter, as well as the operation oftesting the counter, take a number of instructions to perform, an offsetmust be added to the counter in order to compensate for those operationswhich require a number of instructions to execute. Accordingly, ifsetting the counter requires N_(s) instructions and the testing of thecounter N_(t) instructions, Equation 8 becomes:counter value=256−N+N _(s) +N _(t)  Equation 8

N_(s) is added to the counter since the control line 210 has alreadybeen at either HIGH or LOW (depending on where in the pulse 100 sequencethe algorithm is) for N_(s) instructions at the time the counter isstarted. Similarly, N_(t) is added to the counter because by the timethe test is executed on the counter, the control line 210 will havealready been at either HIGH or LOW (depending on where in the pulse 100sequence the algorithm is) for N_(t) instructions.

At block 306, the counter is tested, which operation takes N_(t)instructions, which in turn increases the counter by that number.

Then, at block 308, the result of the test if checked to see if it thecounter has reached or passed 0. If so, the algorithm proceeds to block310. If not, the algorithm repeats the testing operation by going backto block 306.

Once the counter has reached or passed 0, at block 310 the algorithmdetermines if it has to execute additional instructions in order tocompensate for testing operation the offset (N_(t)) that was added insetting the initial value of the counter. This is done because thetesting of the counter is not instantaneous, which means that thecounter may reach or pass 0 within a span of N_(t) instructions. Thusthe counter is being stopped within N_(t) instructions of its desiredvalue so that the next testing operation does not cause the counter toovershoot its desired value of N instructions. This requires from 1 toN_(t) additional instructions to be executed for the count to reach atotal of N instructions. Simple NOP (no operation) instructions may beused for this purpose.

At block 312, the algorithm checks if the control line 210 is presentlyat HIGH. If the control line 210 if not at HIGH, it will go back toblock 302 where it will be set back to HIGH in order to start a newpulse 100 period. If the control line 210 is at HIGH, the algorithmproceeds to block 314 where the control line 210 if set to LOW and thenproceeds to block 304 to complete the ongoing pulse 100 period.

Of course, depending on the implementation and the type ofmicrocontroller used, the offsets may vary in order to provide fordifferent coding structures which vary from one microcontroller type toanother. As well, other timing methods may be used, such as, forexample, a software loop with an interrupt when the counter reaches 0,depending once again on the type of microcontroller used.

The microcontroller 12 uses pulse 100 generated, for example, by thealgorithm depicted in FIG. 5, to synchronize the discharging and turningaround operations of the H-bridge. Building on the example depicted inFIG. 5, FIG. 6 depicts an algorithm that may be executed by themicrocontroller 12 to obtain pulse 100 and synchronize the dischargingand turning around operations of the H-bridge. The sequence of stepscomposing the algorithm is indicated by the sequence of blocks 301 to322.

At block 301, the algorithm sets a pulse period counter to 1, thiscounter is used to count the number of pulse period that have beencompleted.

The sequence of blocks 302 to 314 behaves as described previously forthe flow chart depicted by FIG. 5, with the exception of block 312 wherethe algorithm proceeds to block 316, instead of going back to block 302,whenever the condition of having the pulse set to HIGH is not met.

At block 316, the algorithm checks if the number of pulse period perpolarity has been reached by verifying the value of the pulse periodcounter. If the number of pulse period per polarity has not beenattained, the algorithm proceeds to block 318 where the pulse periodcounter is increased by 1 and then goes back to block 302 to start a newpulse period. On the other hand, if the number of pulse period perpolarity has been attained, the algorithm proceeds to block 320 wherethe H-bridge is discharged followed by block 320 where the H-bridge isturned around. The sequence of N-channel FET activation or deactivationis detailed in the description of FIG. 2. N-channel FETs 140, 150, 160,170 are activated by setting control lines 240, 250, 260, 270,respectively, to HIGH and deactivated by setting those same controllines to LOW. N-channel FETs 120, 130 are activated or deactivated bydeactivating or activating associated N-channel FETs 140, 160respectively. After the H-bridge 19 has been turned around, thealgorithm goes back to block 301 where the pulse period counter is setback to 1 and the process starts over.

To provide for variation in intensity, the microcontroller 12 receivesthe values of T₁, T and the number of pulses per polarity from the mainprocessor 30 through control line 230. The main processor 30 determinesthese values according to the brightness level selected by a user of thescanner 2, using a manual dimmer control 7 on the keyboard 6. Thus theprimary adjustment of the intensity is through the manual dimmer control7 on the keyboard 6.

The adjustability of the intensity may also be used to compensate forother operating conditions or changes in external parameters that may beencountered in use. To compensate for variation in the voltage source 9during use, a voltage level monitor 32 is connected through monitor line34 to the voltage source 9. The main processor 30 adjusts the values ofT₁, T and the number of pulses per polarity according to variations inthe available voltage level as the voltage source 9 discharges.Furthermore, an ambient light level detector 40 may be connected to themain processor 30 through monitor line 36 so that the main processor 30may adjust the values of T₁, T and the number of pulses per polarityaccording to the level of ambient light, e.g. lowering the brightnesslevel in low ambient light conditions and raising it in high ambientlight conditions. The scanner 2 keyboard 6 may be provided with controlsto enable or disable this feature.

The main processor 30 selects the appropriate values of T₁, T and thenumber of pulses per polarity by accessing, for example, a lookup table,provided with the main processor 30, that correlates the desiredbrightness level and optionally the available voltage level and ambientlight level, to the required values for T₁, T and the number of pulses.These values are then transmitted to the microprocessor through controlline 230.

When the main processor 30 sets control line 230 to LOW, themicrocontroller 12 discharges the EL display 14 by enabling theappropriate FETs and the microcontroller 12 is put to sleep. Currentconsumption of the circuit 10 may be reduced if the microcontroller 12discharges the EL display 14 for a few ms and then turns OFF N-channelFETs 140, 160 so that they do not allow current to flow through anyH-bridge resistors 124, 134, 144, 164.

Although the present invention has been described by way of a particularembodiment thereof, it should be noted that modifications may be appliedto the present particular embodiment without departing from the scope ofthe present invention and remain within the scope of the appendedclaims.

1. A control circuit to vary the intensity of an electro luminescentdisplay, said circuit being connected between a power source and adisplay, said circuit comprising a pair of conductors to be connected tosaid display for applying a voltage thereto, a voltage generator, aswitch to control application of said voltage from said generator tosaid conductors, a gating circuit to connect selectively one or other ofsaid conductors to said voltage source, and a controller operating uponsaid switch to vary the duty cycle thereof and upon said gating circuitto alternate periodically the relative polarity of said conductors.
 2. Acontrol circuit according to claim 1 wherein said voltage sourceincludes an inductor and said switch controls current flow through saidinductor.
 3. A control circuit according to claim 1 wherein said dutycycle is variable in response to changes in external parameters.
 4. Acontrol circuit according to claim 3 wherein said duty cycle is variableby manipulation of a manual control.
 5. A control circuit according toclaim 3 wherein said duty cycle is variable in response to variations inambient light.
 6. A control circuit according to claim 3 wherein saidduty cycle is variable in response to variations in said power source.7. A control circuit according to claim 3 wherein said controllerincludes a microprocessor and said duty cycle is determined by a look-uptable operably connected to said microprocessor and monitoring saidexternal parameters.
 8. A control circuit according to claim 7 wherein acounter is set to determine the periods of said duty cycle andincremented by said microprocessor.
 9. A control circuit according toclaim 8 wherein said counter is offset to compensate for operation ofsaid microprocessor.
 10. A control circuit according to claim 1 whereinsaid gating circuit includes a selectively operable discharge path fromeach of said conductors.
 11. A control circuit according to claim 10wherein said controller enables said discharge path as said polarity isreversed.
 12. A control circuit according to claim 11 wherein saiddischarge path includes a current limiting element.